[/
  Copyright 2011 - 2020 John Maddock.
  Copyright 2013 - 2019 Paul A. Bristow.
  Copyright 2013 Christopher Kormanyos.

  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE_1_0.txt or copy at
  http://www.boost.org/LICENSE_1_0.txt).
]

[section:internals Internal Support Code]

There are some traits classes which authors of new backends should be aware of:

   namespace boost{ namespace multiprecision{ namespace detail{

   template<typename From, typename To>
   struct is_explicitly_convertible;

   }}}

Inherits from `boost::integral_constant<bool,true>` if type `From` has an explicit conversion from `To`.

For compilers that support C++11 SFINAE-expressions this trait should "just work".  Otherwise it inherits
from `boost::is_convertible<From, To>::type`, and will need to be specialised for Backends that have
constructors marked as `explicit`.

   template <class From, class To>
   struct is_lossy_conversion
   {
      static const bool value = see below;
   };

Member `value` is true if the conversion from `From` to `To` would result in a loss of precision, and `false` otherwise.

The default version of this trait simply checks whether the ['kind] of conversion (for example from a floating-point to an integer type)
is inherently lossy.  Note that if either of the types `From` or `To` are of an unknown number category (because `number_category` is not
specialised for that type) then this trait will be `true`.

   template<typename From, typename To>
   struct is_restricted_conversion
   {
      static const bool value = see below;
   };

Member `value` is `true` if `From` is only explicitly convertible to `To` and not implicitly convertible, or
if `is_lossy_conversion<From, To>::value` is `true`.  Otherwise `false`.

Note that while this trait is the ultimate arbiter of which constructors are marked as `explicit` in class `number`,
authors of backend types should generally specialise one of the traits above, rather than this one directly.

   template <class T>
   is_signed_number;
   template <class T>
   is_unsigned_number;

These two traits inherit from either `boost::mpl::true_` or `boost::mpl::false_`, by default types are assumed to be signed unless
`is_unsigned_number` is specialized for that type.

[endsect]
